1.Field of the Invention
The present invention relates to a method of and apparatus for producing dilute hydrofluoric acid of ultrahigh purity which is useful as an etchant and cleaning agent for semiconductors.
2.Description of the Prior Art
The purity of chemicals for use in the wet process for producing semiconductors governs the properties of wafers, so that chemicals of higher quality are essential in providing semiconductor devices of higher packing density. Especially, dilute hydrofluoric acid and ultrapure water are most important for use in etching or cleaning in the final step of the wet process, as is well known.
Among other chemicals, hydrofluoric acid is presently purified to a superhigh level by techniques which have made the most remarkable progress, and the impurity concentration of the acid has reached the level of 0.1 ppb. Development attempts are under way to reduce the concentration to the order of parts per trillion (ppt).
However, the use of hydrofluoric acid presently involves storage of hydrofluoric acid of high concentration (usually 50% HF) in a container, transport to a storage tank, transfer from the tank to diluting equipment through piping, dilution of the acid usually to 0.1 to 0.5% HF and supply of the diluted acid to the point of use.
During the entire handling procedure including storage, transport, dilution and supply, the acid is contaminated with the impurities dissolving out from the piping, pump, filter, etc. into the acid and with the contaminants otherwise introduced thereinto and is thereby degraded to several ppb or possibly to hundreds of ppb in terms of impurity content when the acid is delivered to the site of use. Thus, although hydrofluoric acid of very high purity is developed as a product, the high purity is in no way available when the acid is to be used actually.